Fixes error in calculating transition string

Due to a mistake in one of the test cases, the transition ordinals were not calclated correctly.
The test and the code was fixed.

Code refactor and documentation improvements

The code for the libary file and its test were refactored.
No changes in functionality occured in this release.
Go doc comments were slightly improved and fixed.

Offset conversion and documentation

This release adds another API method to the library.
It is a convenience method for converting the standard offset gotten from the go time.Time Zone() function into the POSIX offset used to create the format.

The release also expands on the go doc comments.

Initial release

This is the inital release. The library API is small and documented in the readme file. Please check it out and raise an issue on github if you find any bugs.
Made possible by the release of Go 1.19